Make mobs chase a player

I know one of the demo games green things will chase a player. However I am looking to do something different. My game is a hide a seek / shooting game. The player must find object on the map to continue to a new level. However these objects are gunna be guarded by monsters that my design team has created.

I am looking to make the monsters stand still guarding the object until the player gets within their line of sight. Then I want them to attack the user. Is this possible to do or no? If so can I please be pointed in the right direction?

Look at the logic in the turret demo level as a starting point. When you enter the radius, the turrets will attack the player.

Ok thank you for your insight. This was most helpful.